Question:
What is the common difference of an AP in which $a_{27}-a_{7}=84 ?$
Solution:
$a_{27}=a+26 d$
$a_{7}=a+6 d$
And
$a_{27}-a_{7}=84$
$\Rightarrow a+26 d-a-6 d=84$
$\Rightarrow 20 d=84$
$\Rightarrow d=\frac{21}{5}=4.2$
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